Ratio of turbulent-to-total pressure (color coded) as a function of the effective temperature and the optical depth throughout part of the evolution of a 1.9 M_⊙ model. Extended white areas are radiatively stable. The HeCZ is visible in the top right section of the plot. For T_eff> 7700 K the HCZ is present from the surface up to log (τ) ≈ 2. At lower T_eff  the outer boundary of the HCZ is located at log (τ) ≈ 0, while the inner boundary reaches log (τ) > 3.5 and merges with the HeCZ.